1. Technical Field of the Invention
This disclosure relates generally to signal amplification and in particular, but not exclusively, to image sensor readout circuitry.
2. Background Art
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal oxide semiconductor (CMOS) image sensors, has continued to advance at a great pace. For example, the demands of higher resolution, high quality images and lower power consumption have encouraged the further miniaturization and integration of these CMOS image sensors. However, fixed pattern noise (FPN) is a known issue for CMOS image sensors. FPN is a spatial variation in pixel outputs under uniform illumination due to device and interconnect mismatches within an image sensor. FPN may present itself in a resultant image as some pattern of brighter or dimmer pixels occurring with images taken under the same temperature and exposure.
Conventional methods of reducing column FPN in image sensors include correlated double sampling (CDS), in which a reference signal (or black signal level) is amplified and sampled before resetting a pixel cell. During subsequent image acquisition, the reset pixel cell is exposed to light and charged to produce an image signal. The image signal is amplified, sampled and compared with the sampled reference signal (i.e., black signal is subtracted from the image signal) to arrive at a final value (i.e., the resultant image signal). Accuracy of CDS is limited due to non-linearity and variation of column sampling circuits. As a result, there is normally still an amount of column FPN left even after methods such as CDS is applied.
One source of FPN is capacitor mismatch induced gain error. As pixel cell size continues to decrease in successive generations of image sensor devices, the contribution of capacitor mismatch to induced gain error is becoming more significant.